1. Technical Field
This invention relates to a catalyst arrangement for the exhaust system of an internal combustion engine and more particularly, but not exclusively, it is intended for use in the exhaust system of diesel engines.
2. Description of the Prior Art
In order to reduce the particulate emission levels, it is known to place a particulate filter trap in the exhaust system which captures a high proportion of the particulate material in the exhaust gas. The particulates are collected in a layer on the filter medium and consist predominantly of carbon and hydrocarbon material. The exhaust gas passes through the filter, through the exhaust system, and to atmosphere with a reduced quantity of particulates.
The particulate material builds up in the trap and restricts the flow of exhaust gas and generates a back-pressure on the engine. A condition is reached where regeneration of the particulate trap is required which involves burning off of the deposit in a controlled way. This is achieved by increasing the temperature of the exhaust gas until combustion of the deposit is initiated. Burning continues until all, or most, of the particulate deposit is removed. The increase in exhaust temperature is achieved by operating the engine in a manner that causes a temperature increase (see U.S. Pat. No. 4,404,796) or by using an additional external heating device (see U.S. Pat. No. 4,544,388).
The temperature at which the particulate deposit begins to burn can be influenced by introducing a catalyst powder into the exhaust gas stream (see British Pat. Nos. 2,145,349 and 2,098,086) or by incorporating a catalyst coating on a support element adjacent the filter element (see U.S. Pat. Nos. 4,505,106 and 4,449,362, and British Pat. No. 2,134,407).
Unfortunately, the use of catalysts in this manner is not replenishable and, after consumption, requires complicated replacement of major portion of the regenerative trap.